According to application of water quality standards based on total effluent regulations and environmental impact assessment (EIA) systems, current disposal water levels of a sewage treatment plant are strictly controlled. Existing wastewater treatment systems are substantially insufficient to satisfy such levels. Therefore, there is a strong requirement for development into improved biological filtration processes and distribution thereof capable of satisfying desired water quality standards.
A biological filtration process refers to a process of simultaneously performing biological treatment and filtration, in order to attain excellent treatment effects. Compared to a suspended growth process, the biological filtration process may achieve stable treatment efficiencies at relatively high organic loading and hydraulic load. These results are obtained because microorganisms are deposited on a medium and not washed out even when applying a hydraulic load thereto, instead, having a solid content of 1.0 to 4.0 kg/m3 in the medium so as to maintain high concentration of microorganisms.
The solid content of the medium corresponds to 3,000 to 5,000 mg/L in terms of solid content in a reactor used for a suspended growth process, which is substantially similar to the solid content in a reactor for a suspended growth process of 3,500 mg/L. However, solids contained in the medium mostly comprise active microorganisms, thus having activity of 2 to 3 times that of solids generated during the suspended growth process. For this reason, the biological filtration process needs a plant site with considerably decreased area equal to ½ to ⅓ of a site area for establishment of a suspended growth plant.
FIG. 1 shows a typical one-stage single-medium biological filtration process. Such one-stage single-medium biological filtration process encounters rapid head loss when a solid loading is high, requiring frequent backwashing. A load of soluble solids generally ranges from 3 to 4 kgSS/m3/d and, in case of a normal operation, the solids need backwashing once a day. However, this operation may be impaired if a concentration of solids contained in wastewater inflowing to a biological filter (‘biofilter’) exceeds 100 mg/l. When the solid content of influent water is increased, frequent backwashing is required. However, such a frequent backwashing often causes operation shutdown, increase in backwash water, excessive desorption of microorganisms from the medium which in turn reduces treatment efficiencies, and causes other problems.
FIG. 2 shows a two-stage single-medium biological filtration process disclosed in Korean Patent Registration No. 0817882. Thanks to two stages, more excellent water quality may be accomplished and various operation methods may be embodied. However, when a solid loading is increased, it may cause higher head loss than that generated during one-stage single-medium biological filtration. A medium volume V required for removing pollutants is substantially equal in both the one-stage biological filtration process and the two-stage biological filtration process. However, for one-stage biological filtration, a medium depth H is divided into two equal sections and the lower section (H/2) is operated under anoxic conditions while the upper section (H/2) is operated under aerobic conditions. Therefore, a medium volume of each of the lower and upper sections is ½(V/2). In other words, both these sections have substantially the same medium volume (V=V/2+V/2). In this regard, when the area of a one-stage biofilter is A, a total medium volume may be calculated by the following equation:Vlower (anoxic)=A×H/2=AH/2Vupper (aerobic)=A×H/2=AH/2Vtotal=Vlower (anoxic)+Vupper (aerobic)=AH/2+AH/2=AH 
Herein, a filtering speed LV is defined by flow rate/filtering area (Q/A).
For a two-stage biological filtration process to treat the same wastewater as used in the foregoing one-stage process, a medium volume is substantially identical to that of the one-stage process. However, the filter is divided into two sheets and a volume of one sheet is ½ a volume of the one-stage biofilter.Vtotal=Vfast stage (anoxic)+Vsecond stage (aerobic)=V/2+V/2
Herein, each of an anoxic bath and an aerobic bath has a medium depth of H, and therefore, one sheet of the two-stage biofilter has an area of ½ the area of the one-stage biofilter.Vfirst stage (anoxic)=Vsecond stage (aerobic)=V/2=H×A/2=AH/2
This means that a first stage (anoxic) of the two-stage biofilter has a medium volume substantially equal to that of the lower section (anoxic) of the one-stage biofilter whereas a medium depth of the two-stage biofilter is about 2 fold that of the one-stage biofilter, thereby decreasing an area required for filtering to ½ that of the one-stage biofilter. Such decrease in area of the filter results in a 2 fold-increase in a filtering speed when wastewater inflows at the same flow rate of Q. Since head loss is in proportion to the square of flow velocity (hL=υ2/2 g), such a head loss may be increased 4 fold if a flow velocity of the two-stage biofilter is 2 fold higher than that of the one-stage biofilter. Accordingly, the two-stage filtration process may ensure high quality of treated water but often encounters a problem of increased head loss. Additionally, a first filter of the two-stage biofilter shows 2-fold increase in solid loading.Solid loading of a one-stage biofilter (kgSS/m2/d)=SS content of water inflow×inflow rate×1/A Solid loading of a first filter among two-stage biofilter (kgSS/m2/d)=SS content of water inflow×inflow rate×1/(A/2)=SS content of water inflow×inflow rate×2/A 
In order to solve the foregoing problem regarding head loss caused by solid loading, a multimedia biological filtration process as shown in FIG. 3 has been developed (Korean Patent Registration No. 0591541). Such a multimedia biological filtration process may overcome demerits of the single-medium biological filtration process known in the art.
For purpose of applying the foregoing advantages to secure excellent water quality and to treat initial rainwater, a two-stage multimedia-single medium biological filtration process as shown in FIG. 4 was invented (Korean Patent Registration No. 0794336). However, this process entails disadvantages in that head loss rapidly increases due to chemical sludge (solids) formed by a coagulant, if the coagulant is injected into only a second stage to remove phosphorous during filtration and simple multimedia biological filtration is conducted in a first stage. As a result, the previous techniques do not sufficiently utilize a beneficial feature of the multimedia biological filtration process that can prevent head loss as well as a characteristic of the two-stage biological filtration process that achieves high quality of treated water. In spite of the term ‘multimedia biological filtration process’, the foregoing process substantially consists of two stages and cannot have multiple stages. In fact, when solid loading increases, arranging the media in multi-layers such as large-middle-small size in order may minimize head loss and also secure excellent treated water quality. Moreover, a filtering time is extended which in turn reduces backwashing, thereby decreasing operation costs.